U.S. patent application number 13/639327 was filed with the patent office on 2013-01-24 for temperature sensor element, method for manufacturing same, and temperature sensor.
This patent application is currently assigned to KABUSHIKI KAISHA TOYOTA CHUO KENKYUSHO. The applicant listed for this patent is Tsunenobu Hori, Takao Kobayashi, Kaoru Kuzuoka, Chiaki Ogawa, Motoki Satou, Katsunori Yamada. Invention is credited to Tsunenobu Hori, Takao Kobayashi, Kaoru Kuzuoka, Chiaki Ogawa, Motoki Satou, Katsunori Yamada.
Application Number | 20130020670 13/639327 |
Document ID | / |
Family ID | 44861591 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130020670 |
Kind Code |
A1 |
Hori; Tsunenobu ; et
al. |
January 24, 2013 |
TEMPERATURE SENSOR ELEMENT, METHOD FOR MANUFACTURING SAME, AND
TEMPERATURE SENSOR
Abstract
A temperature sensing element includes a thermistor composed of
Si-base ceramics and a pair of metal electrodes bonded onto the
surfaces of the thermistor. The metal electrodes contain Cr and a
metal element .alpha. having a Si diffusion coefficient higher than
that of Cr. A diffusion layer is formed in a bonding interface
between the thermistor and each metal electrode, the diffusion
layer including a silicide of the metal element .alpha. in a
crystal grain boundary of the Si-base ceramics. A temperature
sensor including the diffusion layers is provided. Owing to the
diffusion layers, the temperature sensor ensures heat resistance
and bonding reliability and enables temperature detection with high
accuracy in a temperature range, in particular, of from -50.degree.
C. to 1050.degree. C.
Inventors: |
Hori; Tsunenobu;
(Kariya-shi, JP) ; Kuzuoka; Kaoru; (Toyota-shi,
JP) ; Ogawa; Chiaki; (Tajimi-shi, JP) ; Satou;
Motoki; (Okazaki-shi, JP) ; Yamada; Katsunori;
(Nagoya, JP) ; Kobayashi; Takao; (Seto-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hori; Tsunenobu
Kuzuoka; Kaoru
Ogawa; Chiaki
Satou; Motoki
Yamada; Katsunori
Kobayashi; Takao |
Kariya-shi
Toyota-shi
Tajimi-shi
Okazaki-shi
Nagoya
Seto-shi |
|
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
KABUSHIKI KAISHA TOYOTA CHUO
KENKYUSHO
Nagakute-shi, Aichi-ken
JP
DENSO CORPORATION
Kariya-city, Aichi-pref.
JP
|
Family ID: |
44861591 |
Appl. No.: |
13/639327 |
Filed: |
April 27, 2011 |
PCT Filed: |
April 27, 2011 |
PCT NO: |
PCT/JP2011/060310 |
371 Date: |
October 4, 2012 |
Current U.S.
Class: |
257/467 ;
257/E21.002; 257/E29.347; 438/54 |
Current CPC
Class: |
H01C 7/021 20130101;
G01K 7/22 20130101; H01C 7/008 20130101 |
Class at
Publication: |
257/467 ; 438/54;
257/E29.347; 257/E21.002 |
International
Class: |
H01L 35/14 20060101
H01L035/14; H01L 35/34 20060101 H01L035/34 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 28, 2010 |
JP |
2010-104413 |
Apr 6, 2011 |
JP |
2011-084154 |
Claims
1. A temperature sensing element comprising: a thermistor that has
electrical characteristics which change with temperature; and a
pair of metal electrodes that are bonded onto a surface of the
thermistor, wherein: the thermistor is composed of Si-base ceramics
including silicon nitride which is a matrix component and silicon
carbide which is contained in the silicon nitride; the pair of
metal electrodes include Cr and a metal element .alpha. that has a
Si diffusion coefficient higher than that of Cr; and a diffusion
layer, in which a silicide of the metal element .alpha. is present
in a crystal grain boundary of the Si-base ceramics that configures
the thermistor, is formed in an interface between the thermistor
and the pair of the metal electrodes.
2. The temperature sensing element according to claim 1, wherein,
in the diffusion layer, the silicide of the metal element .alpha.
and a Cr silicide are present in the crystal grain boundary of the
Si-base ceramics.
3. The temperature sensing element according to claim 1, wherein:
the temperature sensing element includes crystal grains of the
silicon nitride, a crystal grain boundary composed of a glass phase
which is arranged around the crystal grains, and grains of silicon
carbide dispersed in the crystal grain boundary.
4. The temperature sensing element according to claim 3, wherein
the silicide of the metal element .alpha. and the Cr silicide are
arranged, reacting with the grains of the silicon carbide dispersed
in the crystal grain boundary.
5. The temperature sensing element according to claim 1, wherein
the metal element .alpha. is Fe.
6. The temperature sensing element according to claim 1, wherein
the pair of metal electrodes are composed of an alloy that contains
30 to 90 mass % Cr and 10 to 70 mass % Fe.
7. The temperature sensing element according to claim 1, wherein
the pair of the metal electrodes have a linear expansion
coefficient of 11.times.10.sup.-6/.degree. C. or less.
8. The temperature sensing element according to claim 1, wherein
the pair of metal electrodes have a thickness of 3 to 110
.mu.m.
9. The temperature sensing element according to claim 1, wherein
the diffusion layer has a thickness of 3 to 110 .mu.m.
10. A method of manufacturing a temperature sensing element
according to claim 1, comprising: bonding a metal electrode, which
contains Cr and a metal element .alpha. having a Si diffusion
coefficient higher than that of Cr, to a thermistor composed of
Si-base ceramics by using a step of conducting heat treatment under
a condition that a metal configured by the metal electrode is
located on a surface of the thermistor; and forming a diffusion
layer in which a silicide of the metal electrode .alpha. is present
by diffusing the metal element .alpha. into a crystal grain
boundary of the Si-base ceramics, in a bonding interface between
the thermistor and the metal electrode.
11. The method of manufacturing a temperature sensing element
according to claim 10, wherein the metal configuring the metal
electrode is an alloy powder having an average grain size of 50
.mu.m or less.
12. The method of manufacturing a temperature sensing element
according to claim 10, wherein the heat treatment is conducted in a
vacuum or in an atmosphere of an inactive gas.
13. The method of manufacturing a temperature sensing element
according to claim 10, wherein the heat treatment is conducted with
application of pressure and/or voltage.
14. A temperature sensor wherein the temperature sensor comprises
the temperature sensing element according to claim 1.
15. The temperature sensor according to claim 14, wherein the
temperature sensor comprises: the temperature sensing element; a
signal line that is electrically connected to the temperature
sensing element on a tip-end side and is electrically connected to
an external circuit on a rear-end side; and a sheath pin
accommodating the signal lines inside.
16. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention relates to a temperature sensing
element used for detecting a temperature such as of an exhaust gas
and a method of manufacturing the temperature sensing element, and
to a temperature sensor.
BACKGROUND ART
[0002] Generally, temperature sensors are used for measuring a
temperature of an exhaust gas. Such a temperature sensor may be
used for detecting a temperature such as of an exhaust gas that
flows through a flow path. For example, such a temperature sensor
may be used being arranged in a catalytic converter of an exhaust
gas purification system or an exhaust pipe of a vehicle.
[0003] An example of such a temperature sensor is shown in FIG. 9.
In FIG. 9, a temperature sensor 9 includes a temperature sensing
element 90 composed of a thermistor 91 having electrical
characteristics that change with temperature and a pair of metal
electrodes 92 formed on the surfaces of the thermistor 91, and
includes a sheath pin 95 incorporating signal lines 93 which are
connected, on a tip-end side, to the temperature sensing element 90
and electrically connected, on a rear-end side, to an external
circuit.
[0004] In a temperature sensor having such a configuration, the
signal lines 93 are bonded, such as by welding, to the respective
metal electrodes 92 which are bonded to the respective surfaces of
the thermistor 91. Thus, the electrical characteristics of the
thermistor 91 are detected by the external circuit.
[0005] A temperature sensor used in a high-temperature oxidizing
atmosphere, such as in a catalytic converter or an exhaust pipe,
has problems to be solved, i.e. of ensuring bonding reliability of
the temperature sensing element that is a bonded member composed of
the metal electrodes and the thermistor and of ensuring heat
resistance of the metal electrodes.
[0006] As a measure against these problems, a bonded member
composed of ceramic and metal is suggested (see PTL 1). This bonded
member includes a metal film bonded to a surface of a ceramic
material and a surface layer (oxide layer) formed on a surface of
the metal film. PTL 1 also suggests a bonding method that includes
diffusion bonding to achieve bonding between the metal film and the
ceramic. PTL 1 teaches that the bonded member obtained in this way
ensures its heat resistance and bonding reliability.
[0007] On the other hand, another bonded member is suggested, which
is composed of ceramic and metal electrodes bonded to the surfaces
of the ceramic (see PTL 2). The metal electrodes are provided as a
continuous body and have a plurality of recesses. Similar to PTL 1,
PTL 2 suggests a bonding method that includes diffusion bonding.
PTL 2 teaches that the bonded member obtained in this way is able
to reduce thermal stress which is ascribed to the difference in
linear expansion coefficient between the ceramic and the metal
electrodes and thus is able to ensure bonding reliability.
[0008] Further, a ceramic sensor is suggested, which includes a
ceramics plate and a metal electrode bonded to at least one surface
of the ceramics plate (see PTL 3). The metal electrode has an outer
periphery which is entirely or partially cut off to entirely or
partially expose an end of the ceramics plate. Also, at least a
part of the outer periphery of the metal electrode has a thickness
smaller than a center portion thereof. PTL 3 teaches that the
ceramic sensor configured in this way is able to prevent the metal
electrode from being separated.
[0009] Still another bonded member is suggested (see PTL 4 or 5).
This bonded member is obtained by diffusing components of ceramics
and metal into a bonding interface to ensure bonding reliability
between the ceramics and the metal.
[0010] For example, PTL 4 suggests that, in bonding metal that
contains Cr and Fe to nitride-base ceramics, the components
contained in the ceramics are partially diffused into the metal to
enhance bonding reliability. PTL 5 suggests that, in bonding
ceramics, such as silicon nitride or silicon carbide, to metal that
contains Cr and Ni, a silicide of Cr is formed in the interface
between the ceramics and the metal to enhance bonding
reliability.
CITATION LIST
Patent Literature
[0011] [PTL 1] JP-A-2005-343768 [0012] [PTL 2] JP-A-2007-022893
[0013] [PTL 3] JP-A-2009-007206 [0014] [PTL 4] JP-A-S60-180968
[0015] [PTL 5] JP-A-S62-171979
SUMMARY
Technical Problem
[0016] However, due to the recent trend of small-size and
high-power engines, exhaust gases tend to have higher temperatures.
Under such conditions, a problem has come to light. The problem is
that application of a bonded member or a ceramic sensor of a
conventional configuration to a temperature sensor used under
high-temperature conditions lowers the degree of bonding
reliability to cause separation near the bonding interface. This
problem is particularly serious in a bonded member composed of
ceramic and metal, the ceramic being silicon nitride, silicon
carbide or the like having a low linear expansion coefficient.
[0017] Further, a bonded member or a ceramic sensor of a
conventional configuration is not able to ensure ohmic contact
characteristics which are important as a temperature sensor.
[0018] Specifically, in order to detect a temperature with high
accuracy, the resistance of a thermistor detected by an external
circuit is required not to be varied. Further, the resistance value
of the interface between the thermistor and each of the metal
electrodes and the resistance value of the metal electrodes are
required to be extremely small and the bonding areas are required
to be uniform.
[0019] For example, in the technique disclosed in PTL 1, resistance
of the thermistor may be varied due to the influence of the oxide
layer. In the technique disclosed in PTL 2, the dimensions of the
recesses may directly influence the bonding areas and thus
non-uniformity of the bonding areas is unlikely to be
suppressed.
[0020] The technique disclosed in PTL 4 or 5 aims at enhancing only
the bonding reliability. Therefore, the technique tends to allow
increase and variation in the resistance value and thus is not able
to ensure ohmic contact characteristics which are important as a
temperature sensor.
[0021] Specifically, in the manufacturing method disclosed in PTL 4
or 5, the bonded member is baked at a high temperature of about
1350.degree. C. using HP or HIP to diffuse the components into the
ceramics. Therefore, the resistance value of the metal becomes
large and tends to be varied. When such a bonded member is applied
to a temperature sensor, the temperature sensor will suffer from a
problem of not being able to ensure ohmic contact characteristics
which are important as a temperature sensor.
[0022] Moreover, ceramics that contains silicon nitride or silicon
carbide as a matrix component generally has a crystal grain
boundary area which is very small compared to a crystal area.
Therefore, diffusion from the metal into the crystal grain boundary
of the ceramics is extremely small, causing large variation in a
diffused state.
[0023] Use of such a bonded member as a temperature sensor does not
ensure ohmic contact characteristics which are likely to be
influenced by an oxidized state of the metal and a diffused state
of the bonding interface. In particular, when the temperature
sensor is used covering a large temperature range of -50.degree. C.
to 1050.degree. C., high measurement accuracy is not achieved due
to the variation in the resistance value. In addition, since heat
resistance is also deteriorated, separation and cracks are easily
caused. Thus, application of such a bonded member to a temperature
sensor is problematically difficult.
[0024] The present invention has been made in light of the problems
set forth above to provide a temperature sensing element that
ensures heat resistance and bonding reliability under
high-temperature conditions, by forming a diffusion layer by
positively diffusing components of a metal electrode into a crystal
grain boundary of a thermistor, in a bonding interface between the
thermistor and the metal electrode of the temperature sensing
element, a method of manufacturing the temperature sensing element,
and a temperature sensor.
Solution to Problem
[0025] A first invention provides a temperature sensing element
including a thermistor having electrical characteristics that
change with temperature and a pair of metal electrodes bonded onto
surfaces of the thermistor, in which: the thermistor is composed of
Si-base ceramics and the metal electrodes include Cr and a metal
element .alpha. that has a Si diffusion coefficient higher than
that of Cr; and a diffusion layer is formed in an interface between
the thermistor and each of the metal electrodes, the diffusion
layer including a silicide of the metal element .alpha. in a
crystal grain boundary of the Si-base ceramics that composes the
thermistor.
[0026] A second invention provides a method of manufacturing a
temperature sensing element, including: bonding a metal electrode,
which contains Cr and a metal element .alpha. having a Si diffusion
coefficient higher than that of Cr, to a thermistor composed of
Si-base ceramics by using a step of conducting heat treatment under
a condition that a metal configured by the metal electrode is
located on a surface of the thermistor; and forming a diffusion
layer in which a silicide of the metal electrode .alpha. is present
by diffusing the metal element .alpha. into a crystal grain
boundary of the Si-base ceramics, in an interface between the
thermistor and the metal electrode.
[0027] A third invention provides a temperature sensor that
includes the temperature sensing element according to the first
invention.
Advantageous Effects of Invention
[0028] In the temperature sensing element, the method of
manufacturing the same and the temperature sensor according to the
first to third inventions, the thermistor is composed of Si-base
ceramics, and accordingly, ensures high heat resistance as a
temperature sensor. The metal element Cr contained in each of the
metal electrodes reduces the difference in linear expansion
coefficient between the electrode and the thermistor. For example,
in a heat treatment at a baking temperature of 1200.degree. C., the
metal element Cr is able to suppress crack generation that is
caused in an interface between the electrode and the
thermistor.
[0029] The metal element .alpha. contained in the metal electrodes
has a Si diffusion coefficient higher than that of Cr, and
therefore, produces a silicide compound much easier than Cr. The
silicide of the metal element .alpha. has a linear expansion
coefficient smaller than that of a Cr silicide. Also, the silicide
of the metal element .alpha. has a resistance value smaller than
that of the Cr silicide. Accordingly, the metal element .alpha.
preferentially diffuses into the crystal grain boundary of the
thermistor to form the silicide of the metal element .alpha.,
thereby forming the diffusion layer of the present invention. The
diffusion layer ensures heat resistance and bonding reliability
under high temperature conditions. The diffusion layer also ensures
ohmic contact to suppress variation in the resistance value. In
particular, owing to the diffusion layer, substantially a uniform
resistance value is ensured in a wide temperature range of
-50.degree. C. to 1050.degree. C., thereby realizing a temperature
sensor that enables temperature detection with high accuracy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a partial front cross-sectional view illustrating
a temperature sensor according to the present invention;
[0031] FIG. 2 (a) shows a perspective view of a tip end portion of
the temperature sensor illustrated in FIG. 1, and FIGS. 2 (b) and
(c) show a perspective view of another example of the tip end
portion of the temperature sensor shown by FIG. 2 (a);
[0032] FIG. 3 is a cross-sectional view illustrating a temperature
sensing element in the temperature sensor illustrated in FIG.
1;
[0033] FIG. 4 is an explanatory view illustrating an internal
structure of a thermistor of the temperature sensing element of the
present invention;
[0034] FIG. 5 is an explanatory view illustrating Si diffusion
coefficient of various metal elements;
[0035] FIG. 6 is an explanatory view illustrating heat-resistant
temperature and linear expansion coefficient of metals and metal
silicides thereof;
[0036] FIG. 7 shows explanatory views illustrating a method of
manufacturing the temperature sensing element of the present
invention, specifically (a) shows an explanatory cross-sectional
view of a thermistor coated with a paste of alloy powder, (b1)
shows a partially enlarged view of the coating shown by (a), (b2)
shows an explanatory view of a state where heat treatment causes
sintering in a metal electrode and diffusion of components of the
metal electrode, and (b3) shows an explanatory partial
cross-sectional view around a diffusion layer in the thermistor
after the heat treatment, the thermistor being formed with metal
the electrode and the diffusion layer;
[0037] FIG. 8 (a) shows a cross-sectional view of a first
embodiment of the temperature sensing element according to the
present invention, and FIG. 8 (b) shows a cross-sectional view of a
second embodiment of the temperature sensing element according to
the present invention;
[0038] FIG. 9 is a partial front cross-sectional view illustrating
a temperature sensor according to conventional art;
[0039] FIG. 10 (a) shows an ADF-STEM image indicating a diffusion
layer according to an example of the temperature sensing element of
the present invention, FIG. 10 (b) shows an explanatory view
indicating an analysis of Cr element in the image shown by FIG. 10
(a), and FIG. 10 (c) shows an explanatory view indicating an
analysis of a metal element .alpha. (Fe element) in the image shown
by FIG. 10 (a);
[0040] FIG. 11 is a model diagram illustrating an appearance of the
diffusion layer in a boundary portion between a metal electrode and
a thermistor according to the example of the temperature sensing
element of the present invention;
[0041] FIG. 12 is a model diagram illustrating an appearance of a
diffusion layer in a boundary portion between a metal electrode and
a thermistor according to a comparative example of the temperature
sensing element of the present invention;
[0042] FIG. 13 is an explanatory view illustrating a relationship
of rate of oxidation increase (%) before and after a
high-temperature exposure test (heat resistance evaluation) with
respect to Fe-mixing rate (mass %) in a Cr--Fe alloy, according to
a third experimental example;
[0043] FIG. 14 is an explanatory view illustrating a relationship
of Al-mixing rate in a metal electrode with respect to rate of
oxidation increase (%) before and after the high-temperature
exposure test, according to the third experimental example; and
[0044] FIG. 15 (a) shows an explanatory scanning electron
micrograph indicating an interface between a metal electrode and a
thermistor, FIG. 15 (b) shows an explanatory micrograph indicating
an energy dispersive X-ray fluorescence analysis of Si, FIG. 15 (c)
shows an explanatory micrograph indicating an energy dispersive
X-ray fluorescence analysis of Cr, and FIG. 15 (d) shows an
explanatory micrograph indicating an energy dispersive X-ray
fluorescence analysis of Fe, according to a fourth experimental
example.
DESCRIPTION OF EMBODIMENTS
[0045] With reference to FIGS. 1 to 7, hereinafter is described an
exemplary embodiment according to the present invention.
[0046] As shown in FIG. 1, a temperature sensor of the present
example is used as a sensor for measuring the temperature of an
exhaust gas of a vehicle.
[0047] A temperature sensor 5 includes a temperature sensing
element 1 connected thereto on a tip-end side. The temperature
sensing element 1 has a thermistor 10 whose electrical
characteristics change with temperature and a pair of metal
electrodes 11 provided on the surfaces of the thermistor 10. The
temperature sensor 5 also includes a sheath pin 3 that accommodates
a pair of signal lines 21. The pair of signal lines 21 are
connected to an external circuit (not shown), on a rear-end side of
the temperature sensor 5.
[0048] The temperature sensing element 1 which is provided on the
tip-end side is accommodated in a cover 4. The sheath pin 3 has an
outer periphery along which a rib 12 is formed so as to be located
on the rear-end side with reference to the cover 4.
[0049] The rib 12 is provided with: a contact portion 121 which is
in contact with a tip end surface of an inner wall of a boss for
mounting the temperature sensor 5 to an internal combustion engine;
a rearward extension portion 122 extending rearward from the
contact portion and having an outer diameter smaller than that of
the contact portion 121; and a forward extension portion 123
extending forward from the contact portion and having an outer
diameter smaller than that of the contact portion 121. The sheath
pin 3 is inserted and fitted into the contact portion 121, the
rearward extension portion 122 and the forward extension portion
123. The rib 12 is welded to the sheath pin 3 in the forward
extension portion 123 throughout the circumference thereof.
[0050] The rearward extension portion 122 has an outer periphery
along which one end of a protective tube 13 is welded and fixed to
protect a portion of the sheath pin 3 and the signal lines 21.
[0051] The cover 4 is welded, throughout its circumference, to an
outer periphery of a tip end portion 301 of the sheath pin 3. The
sheath pin 3 and the cover 4 are made of stainless steel or Ni-base
heat-resistant alloy. Further, the rib 12 and the protective tube
13 are also made of stainless steel or Ni-base heat-resistant
alloy.
[0052] The sheath pin 3 is provided with: the two signal lines 21
which are made of stainless steel or Ni-base heat-resistant alloy;
an insulating member 33 which is made of an insulating powder, such
as magnesia, and arranged around the signal lines 21; and an outer
tube member 34 which is made of stainless steel or Ni-base
heat-resistant alloy and covers the outer periphery of the
insulating member 33. The sheath pin 3 has a columnar shape, while
the outer tube member 34 has a cylindrical shape. The signal lines
21 are exposed on the tip-end side and the rear-end side from the
insulating member 33 and the outer tube member 34. FIG. 2 (a) shows
a perspective view of a tip end portion of the temperature sensor 5
illustrated in FIG. 1. As shown by FIG. 2 (a), each signal line 21
has a tip end which is bonded to a corresponding metal electrode 11
of the temperature sensing element 1, and a rear end which is
connected to a corresponding external signal line (not shown) which
is further connected to the external circuit.
[0053] As shown by FIG. 2 (b), the temperature sensing element 1
will have no problem if it is not covered with a mold 6. However,
as shown by FIG. 2 (a), a part of the temperature sensing member 1
and the signal lines 21 may preferably be sealed by the mold 6. The
mold 6 is made of materials, such as an inorganic material,
amorphous glass and crystallized glass. These materials have an
effect of protecting the temperature sensing element 1 under a
temperature of 1000.degree. C. or more. If each of these materials
by itself has a desired range of linear thermal expansion
coefficient, the material may be solely used for forming the mold
6. Alternatively, amorphous glass may be mixed with crystallized
glass, or glass may be added with an inorganic material powder, for
example, so as to have a desired linear expansion coefficient and
to be used for forming the mold 6. The inorganic material powder
added to glass may include aluminum oxide (Al.sub.2O.sub.3),
magnesium oxide (MgO), yttrium oxide (Y.sub.2O.sub.3), chromium
oxide (Cr.sub.2O.sub.3), zirconium oxide (ZrO.sub.2), or a
low-thermal-expansion ceramics that configures a thermosensor
2.
[0054] As shown by FIG. 2 (c), each signal line 21 may be connected
to the temperature sensing element 1 via an electrode line 211. In
this case, each electrode line 211 is connected to the
corresponding signal line 21 such as by laser welding. Thus, a
difference in linear expansion coefficient may be further reduced
between the temperature sensing element 1 and the signal line 21.
Also, a configuration that can be easily manufactured may be
provided.
[0055] The temperature sensing element 1 used for the temperature
sensor 5 has a substantially rectangular parallelepiped shaped. As
shown in FIG. 3, in the temperature sensing element 1, the pair of
metal electrodes 11 are formed on the surfaces of the thermistor 10
having electrical characteristics that change with temperature.
Further, a diffusion layer 12 is formed in an interface between the
thermistor 10 and each metal electrode 11.
[0056] The signal lines 21 extending from inside the sheath pin 3
are directly bonded to the respective metal electrodes 11 of the
temperature sensing element 1. The temperature sensing element 1
and the signal lines 21 exposed from the sheath pin 3 on the
tip-end side are inserted into the cover 4 which is welded to the
periphery of the sheath pin 3.
[0057] The thermistor 10 is composed of Si-base ceramics that
contains silicon nitride or silicon carbide as a matrix component.
It is preferable that the thermistor 10 contains silicon carbide in
addition to silicon nitride as a matrix component. The thermistor
10 obtained in this way may exert good mechanical characteristics
and heat resistance. For example, the thermistor 10 that may be
realized is composed such as of low thermal expansion ceramics
having a linear expansion coefficient of about 3.times.10.sup.-6 to
5.times.10.sup.-6/.degree. C. Thus, bonding reliability is
sufficiently ensured between each of the metal electrodes 11 and
the thermistor 10. Containing both of silicon carbide and silicon
nitride, the thermistor 10 used in the temperature sensing element
1 may preferably have a configuration as shown in FIG. 4.
Specifically, the thermistor 10 may preferably have a configuration
including crystal grains 101 composed of silicon nitride, a crystal
grain boundary 105 composed of a crystallized glass phase or a
glass phase and arranged around the crystal grains 101, and silicon
carbide grains 102 and metal conductors 103, which are dispersed in
the crystal grain boundary 105.
[0058] This is because such a configuration is able to form an
electrically conductive path in the crystal grain boundary 105 of
silicon nitride. In this case, the electrically conductive path
makes use of the resistance of the silicon carbide (semiconductor)
itself and the intergrain resistance (tunnel resistance) of the
silicon carbide, as well as its temperature and electrical
characteristics that the resistance changes with temperature. The
thermistor 10 having such a configuration can realize a temperature
sensor which is able to detect a temperature with good sensitivity
in a wide temperature range such as of -80.degree. C. to
1200.degree. C., in particular, -50.degree. C. to 1050.degree. C.
Begin composed of a composite material of ceramics having good heat
resistance, the thermistor 10 is able to exert enhanced heat
resistance. It should be appreciated that the crystal grain
boundary composed of a glass phase recited in the claims refers to
a crystal grain boundary composed of a crystallized glass phase or
a glass phase as mentioned above.
[0059] Further, the crystal grain boundary 105 may preferably be
dispersed with the metal conductors 103. In this case, the
resistance value of the thermistor 10 is easily controlled to a
desired value. For example, the metal conductors 103 may be grains
of silicides, borides, nitrides and carbides of groups 4 to 6 of
the periodic table, such as TiB.sub.2, VN, TiO.sub.2, TiN.sub.2,
CrB.sub.2 and WSi.sub.2.
[0060] The metal electrodes 11 formed on the respective surfaces of
the thermistor 10 contain Cr and a metal element .alpha. that has a
higher Si diffusion coefficient than Cr.
[0061] The difference in linear expansion coefficient between the
metal electrode 11 and the thermistor 10 configured by Si-base
ceramics is reduced by Cr contained in the metal electrodes 11.
Recently, temperature sensors are required to be usable covering an
extremely wide temperature range of about -50.degree. C. to
1050.degree. C. Since Cr is included, the occurrence of cracks is
reduced in the use of the wide temperature range, the cracks being
ascribed to the difference in linear expansion coefficient between
the metal electrode 11 and the thermistor 10.
[0062] Having a higher Si diffusion coefficient than Cr, the metal
element .alpha. may produce a silicide compound much easier than
Cr. In other words, the metal element .alpha. is diffused into the
crystal grain boundary of the thermistor 10 to positively form a
diffusion layer made of the silicide of the metal element
.alpha..
[0063] The metal element .alpha. may be selected such as from Fe,
Mo, Ni, W, Zr, Nb and Ta.
[0064] From the viewpoint of reducing the difference, in linear
expansion coefficient, from the thermistor 10 and of enhancing heat
resistance, the metal electrode 11 is preferably made of an alloy
that contains 30 to 90 mass % of Cr and 10 to 70 mass % of Fe. In
particular, the alloy may more preferably contain 60 mass % of Cr
and 40 mass % of Fe.
[0065] In this way, the metal electrode 11 and the like are
suppressed from being oxidized under high-temperature conditions.
Accordingly, heat resistance of the metal electrode 11 is more
enhanced, while thermal stress is mitigated, which is ascribed to
the difference in linear expansion coefficient between the
thermistor 10 and the metal electrode 11, thereby ensuring bonding
reliability. This is because addition of Fe to Cr allows selective
oxidization of Cr that has low free energy of formation to form a
more uniform oxide film, and thus because oxidization of the Cr--Fe
alloy is suppressed from being advanced. Too much addition of Fe
may not only deteriorate oxidization resistance, but also increase
the linear expansion coefficient of the metal electrodes 11,
thereby widening the difference in linear expansion coefficient
from the thermistor 10. As a result, thermal stress is
increased.
[0066] The alloy may further contain 0.5 to 7 mass % of Al to
suppress the metal electrode 11 from being oxidized under
high-temperature conditions. Thus, heat resistance of the metal
electrode 11 is more enhanced. If the content of Al is less than
0.5 mass %, the effect of enhancing heat resistance is unlikely to
be sufficiently exerted. On the other hand, if the content of Al
exceeds 7 mass %, the effect of enhancing heat resistance is hardly
exerted. On the contrary, the degree of hardness of the metal
electrode 11 tends to be increased and thus to deteriorate
processability thereof.
[0067] FIG. 5 shows Si diffusion coefficient of typical metal
elements. The figure shows a bar chart in which the horizontal axis
indicates metal element and the vertical axis indicates Si
diffusion coefficient (cm.sup.2/s).
[0068] FIG. 6 shows typical metals, as well as heat-resistant
temperatures and linear expansion coefficients of the respective
silicides of the metals. In the figure, the horizontal axis
indicates linear expansion coefficient (.times.10.sup.-6/.degree.
C.) and the vertical axis indicates heat-resistant temperature
(.degree. C.). In the figure, circled areas each indicate a linear
expansion coefficient and a heat-resistant temperature that can be
exhibited by each of the alloys, pure metals and silicides of the
metals.
[0069] The metal electrode 11 may preferably have a linear
expansion coefficient of equal to or less than
11.times.10.sup.-6/.degree. C.
[0070] When the linear expansion coefficient exceeds
11.times.10.sup.-6/.degree. C., use of a low-thermal-expansion
ceramics as the thermistor 10 is likely to make it difficult to
ensure bonding reliability between the metal electrode 11 and the
thermistor 10. When the linear expansion coefficient of the metal
electrode 11 is made excessively low, the temperature sensor, if it
is configured by bonding the signal lines and the like to the
respective metal electrodes 11, will suffer from a large difference
in the linear expansion coefficient between the metal electrode 11
and the signal lines. Therefore, taking into account the possible
increase of thermal stress generated on the bonded interface
between the metal electrode 11 and the corresponding signal line,
the metal electrode 11 may preferably have a linear expansion
coefficient of equal to or more than 7.times.10.sup.-6/.degree.
C.
[0071] Use of a metal (e.g., noble metal, such as Pt) having a low
linear expansion coefficient as the signal lines can further reduce
the linear expansion coefficient of the metal electrodes 11.
However, this may involve a problem of high cost. As a measure
against this, a high-heat-resistant metal, such as Ni--Cr--Fe alloy
or Fe--Cr--Al alloy, may be used as the signal lines, while the
metal electrodes are permitted to have a linear expansion
coefficient of 7.times.10.sup.-6/.degree. C. or more. Use of such
signal lines and metal electrodes is preferable from a viewpoint of
not only obtaining satisfactory bonding reliability but also of
realizing low cost.
[0072] For example, the linear expansion coefficient of the metal
electrodes 11 is controlled by selecting the metal element .alpha.
and adjusting a mixing ratio of the metal element .alpha. to be
contained in the metal electrodes 11 in addition to Cr.
[0073] Each electrode 11 may preferably have a thickness of 3 to
110 .mu.m. When the thickness is less than 3 .mu.m, the strength of
the metal electrode 11 may be insufficient, causing cracks in the
metal electrode 11 due to thermal stress. On the other hand, when
the thickness exceeds 110 .mu.m, the stress due to the difference
in linear expansion coefficient may be caused in the bonding
interface, easily causing crack generation in the thermistor
10.
[0074] In the present invention, it is important that, in the
diffusion layer 12 formed in the interface between the thermistor
10 and each metal electrode 11, the silicide of the metal element
.alpha. which is contained in the metal electrode 11 is present in
the crystal grain boundary of the Si-base ceramics that forms the
thermistor 10.
[0075] As shown in FIG. 3, the diffusion layer 12 enhances the
bonding properties between the thermistor 10 and the corresponding
electrode 11, while ensuring ohmic contact.
[0076] The silicide of the metal element .alpha. has a low linear
expansion coefficient and a low resistance value compared to the
silicide of Cr. In such a relationship, the silicide of the metal
element .alpha. functions as a thermal-stress buffer layer having a
thermal expansion coefficient which is intermediate between the
thermistor 10 and the metal electrode 11. At the same time, the
silicide of the metal element .alpha. serves as the diffusion layer
12 having a low resistance of a level that will not inhibit the
ohmic contact between the thermistor 10 and the metal electrode 11.
Accordingly, the silicide of the metal element .alpha. can enhance
the bonding strength between the thermistor 10 and the metal
electrode 11 and ensure the "ohmic contact" to suppress the
resistance of the temperature sensing element from being
varied.
[0077] In particular, the metal element .alpha. used for composing
the metal electrodes 11 may preferably be Fe. In this case, the
metal element Fe reacts with Si in the crystal grain boundary (Si
contained in the crystallized glass phase or the glass phase and Si
contained in the grains of silicon carbide in the crystal grain
boundary) of the thermistor 10 to form FeSi in the crystal grain
boundary of the thermistor 10. FeSi will have a linear expansion
coefficient which is intermediate between those of the thermistor
10 and the metal electrode 11. Accordingly, FeSi is able to buffer
thermal stress induced by both of the thermistor 10 and the metal
electrode 11 and enhance bonding properties.
[0078] In order to have the silicide of the metal element .alpha.
be present in the crystal grain boundary of the Si-base ceramics
composing the thermistor 10, the Si-base ceramics composing the
thermistor 10 is required to have a large crystal grain boundary
area to provide the thermistor 10 with a crystal structure that
allows easy diffusion of the metal element .alpha.. To this end,
the Si-base ceramics composing the thermistor 10 may preferably
contain both of silicon carbide and silicon nitride.
[0079] Therefore, the silicon carbide grains 102 that easily react
with the metal element .alpha., such as Fe, may be diffused into
the crystal grain boundary 105 in between the crystal grains 101 of
silicon nitride. Thus, the silicon carbide grains 102 are
positively diffused into the crystal grain boundary 105. It is more
preferable that additive amount of silicon carbide is contained by
15 to 50 vol % and that of silicon nitride is contained by 50 to 85
vol %. Allowing the thermistor 10 to have a content of silicon
carbide by 15 to 50 vol %, the silicon carbide grains 102 for
forming the electrically conductive path positively enter in
between the crystal grains 101 of silicon nitride. As a result, the
thermistor 10 will have a large proportion of crystal grain
boundary. The thermistor 10 may additionally contain a component
TiB.sub.2 to trap oxygen at the time of baking. This may allow the
silicide of the metal element .alpha. to be easily present in the
crystal grain boundary.
[0080] The presence or absence of the silicide of the metal element
.alpha. in the crystal grain boundary can be confirmed using a TEM
(Transmission Electron Microscope).
[0081] The diffusion layer 12, as shown in FIG. 10, corresponds to
a region where the silicide of the metal element .alpha. contained
in the metal electrode 11 is mainly present in the crystal grain
boundary of the Si-base ceramics composing the thermistor 10.
[0082] The diffusion layers 12 may each preferably have a thickness
of 3 to 110 .mu.m.
[0083] When each diffusion layer 12 has a thickness of less than 3
.mu.m, the strength of the diffusion layer 12 will become
insufficient and is likely to cause crack generation due to thermal
stress. On the other hand, when the thickness exceeds 110 .mu.m,
the effect of buffering thermal stress, as a function of the
diffusion layer 12, is deteriorated. When the thickness of the
diffusion layer 12 that has a lower strength than the thermistor 10
is increased, the bonding strength may be lowered.
[0084] Hereinafter is described a method of manufacturing the
temperature sensing element of the present invention, according to
an example.
[0085] First, the thermistor 10 is prepared as follows.
[0086] A mixture material was obtained by blending 63.4 vol % of
silicon nitride (Si.sub.3N.sub.4) powder having an average grain
size of 0.7 .mu.m, 30 vol % of silicon carbide (SiC) powder having
an average grain size of 0.2 .mu.m, 6 vol % of yttrium oxide, as a
sintering aid, having an average grain size of 0.5 .mu.m, and 0.6
vol % of TiB.sub.2 powder, as metal conductors, having an average
grain size of 0.4 .mu.m, followed by mixing for 24 hours with
ethanol using a ball mill
[0087] Then, the mixture material was molded by means of uniaxial
pressing at a pressure of 20 MPa, followed by performing hot
pressing for one hour in a N.sub.2 atmosphere at a temperature of
1850.degree. C. and at a pressure of 20 MPa.
[0088] Important baking conditions for increasing the crystal grain
boundary area in a sintered body of the thermistor 10 include:
avoiding insufficient sintering so as not to cause voids that
increase resistance variation; and avoiding excessive sintering by
adjusting temperature, time, atmosphere, applied pressure and the
like so as not to narrow the crystal grain boundary.
[0089] Thus, a parallelepiped (plate-like) sintered body was
obtained as the thermistor 10, with its dimension being 1.0 mm in
depth.times.1.0 mm in width.times.0.5 mm in height.
[0090] Then, as shown in FIGS. 7 (a) and (b1), a Cr--Fe alloy paste
110 that contained Cr by 60 mass % and Fe by 40 mass % was printed
on the surfaces of the thermistor 10 in a thickness of about 100
.mu.m, the surfaces being opposed to each other in the height
direction thereof. The Cr--Fe alloy paste 110 included an alloy
powder 119 having an average grain size (median size D50) of 50
.mu.m or less as measured by a laser-diffraction grain size
distribution measuring device.
[0091] When the alloy powder 119 has an average grain size of 50
.mu.m or less, the thickness of each metal electrode 11 is easily
controlled. When exceeding 50 .mu.m, however, the thickness of each
metal electrode 11 tends to become easily large exceeding 110 .mu.m
and increase thermal stress, resultantly causing cracks.
[0092] Besides printing, the metal electrodes 11 may be formed
using thermal spraying, plating, a thermal transfer sheet, a
dispenser, ink jetting, brush coating, compression molding, vapor
deposition, a metal foil, or the like. From a viewpoint of
workability and adhesion with an even thickness, it may be
preferable that a paste of alloy powder is printed on a transfer
sheet to thereby form each metal electrode 11 on the surface of the
thermistor 10.
[0093] The thickness of the metal electrode 11 can be controlled by
adjusting the amount of metal serving as the metal electrode 11 and
arranged on the thermistor 10.
[0094] Then, heat treatment was conducted under predetermined
conditions to form the metal electrodes 11 and the respective
diffusion layers 12. The heat treatment was conducted by performing
degreasing at a temperature of 400.degree. C. and holding the
resultant body for ten minutes at a temperature of 1150.degree. C.,
while applying pressure of about 30 MPa to the alloy paste 110
formed on the surfaces of the thermistor 10, using SPS (Spark
Plasma Sintering: a sintering process of applying pressure and
current at the time of heat treatment) (see FIGS. 7 (b2) and
(b3)).
[0095] The heat treatment may preferably be conducted at a
temperature of 900 to 1300.degree. C. The thickness of each
diffusion layer 12 can be controlled by adjusting the temperature
and the time of heat treatment. Specifically, the heating
temperature is raised or the heating time is lengthened to advance
diffusion, as 117, of the components of each metal electrode 11
into the thermistor 10. Thus, the thickness of each diffusion layer
12 is increased (see FIGS. 7 (b2) and (b3)).
[0096] The heat treatment may preferably be conducted in a vacuum
or in an atmosphere of an inactive gas, such as nitrogen or argon,
in order to prevent oxidization of the metal electrodes 11.
[0097] Further, the heat treatment may preferably be conducted with
an application of a pressure and/or a voltage. Thus, as shown in
FIGS. 7 (b1) to (b3), sintering properties of the metal serving as
the metal electrodes 11 are enhanced. As a result, the diffusion
117 of the metal element .alpha. into the thermistor 10 is
advanced. Moreover, since the heating temperature can be decreased,
the damages caused by the heating to the thermistor 10 and the
metal electrodes 11 are reduced, and in addition, the heating time
is also shortened.
[0098] In this way, the temperature sensing element 1, as shown in
FIG. 3, was obtained.
[0099] In the temperature sensing element 1, each diffusion layer
12 is formed in the bonding interface between the thermistor 10 and
each metal electrode 11 as a result of the positive diffusion of
the components of the metal electrode 11 into the crystal grain
boundary of the thermistor 10. When the temperature sensing element
1 obtained in this way is used in the temperature sensor 5
described above and shown in FIG. 1, heat resistance and bonding
reliability are ensured under high-temperature conditions. At the
same time, owing to good ohmic contact characteristics with no
variation in the resistance value, substantially balanced
temperature detection is enabled covering a wide temperature range
of -50.degree. C. to 1050.degree. C.
[0100] In a first embodiment of the temperature sensing element,
the Cr--Fe alloy paste 110, for example, is printed first on the
thermistor 10 that has been obtained in a manner similar to the
manufacturing method described above. The Cr--Fe paste 110 is
printed on the surfaces that are opposed to each other in the
height direction of the thermistor, followed by heat treatment (see
FIGS. 7 (b1) and (b2)). Through the heat treatment, the Cr--Fe
alloy paste 110 is all diffused to form each diffusion layer 12
(see FIG. 8 (a)). After that, as shown in FIG. 8 (a), a metal foil
made such as of a Fe--Cr--Al alloy may be bonded to the surface of
the diffusion layer 12 to form the metal electrode 11. Thus, the
diffusion layer 12 is formed with higher reliability and with less
unevenness. In addition, the metal electrode 11 made up of a metal
foil will have high oxidization resistance and exert high bonding
strength when a lead is bonded onto the metal electrode 11.
[0101] In a second embodiment, the Cr--Fe alloy paste 110, for
example, is printed first on the thermistor 10 that has been
obtained in a manner similar to the manufacturing method described
above. The Cr--Fe paste 110 is printed on the surfaces which are
opposed to each other in the height direction of the thermistor 10,
followed by heat treatment (see FIGS. 7 (b1) and (b2)). Through the
heat treatment, a part of the Cr--Fe alloy paste 110 is diffused to
form each diffusion layer 12, with another part thereof being
formed as a first metal electrode 111 (see FIG. 8 (b)). After that,
as shown in FIG. 8 (b), a metal foil made such as of a Fe--Cr--Al
alloy may be bonded to the surface of the first metal electrode 111
to form a second metal electrode 112. Thus, the metal electrode is
formed into a multilayer structure so that the diffusion layer 12
will have higher reliability and less unevenness. In addition, the
metal electrode 11 made up of a metal foil will have high
oxidization resistance and exert high bonding strength when a lead
is bonded onto the metal electrode 11.
[0102] The Fe--Cr--Al alloy mentioned above may be SUH21 (BS G4312)
or the like. SUH21 is made up of 17 to 21 mass % of Cr, 2.0 to 4.0
mass % of Al, 0.10 mass % or less of C, 1.50 mass % or less of Si,
1.0 mass % or less of Mn, 0.040 mass % or less of P, 0.030 mass %
or less of S, and Fe occupying the rest.
EXAMPLES
[0103] Various samples of temperature sensing element were prepared
for evaluation and for confirmation of the effects of the present
invention.
Experimental Example 1
[0104] First, samples X0 and X1 were prepared for evaluation.
[0105] For sample X1, i.e. a sample of the temperature sensing
element of the present invention, the following mixture material
was prepared as the thermistor 10. The mixture material was
prepared by blending 63.4 vol % of silicon nitride
(Si.sub.3N.sub.4) powder having an average grain size of 0.7 .mu.m,
30 vol % of silicon carbide (SiC) powder having an average grain
size of 0.2 .mu.m, 6 vol % of yttrium oxide (Y.sub.2O.sub.3)
powder, as a sintering aid, having an average grain size of 0.5
.mu.m, and 0.6 vol % of TiB.sub.2 powder, as metal conductors,
having an average grain size of 0.4 .mu.m, followed by mixing for
24 hours with ethanol using a ball mill.
[0106] Then, the mixture material was molded by means of uniaxial
pressing at a pressure of 20 MPa, followed by performing hot
pressing for one hour under a N.sub.2 atmosphere at a temperature
of 1850.degree. C. and at a pressure of 20 MPa. Thus, a
parallelepiped (plate-like) sintered body was obtained as the
thermistor 10, with its dimension being 1.0 mm in depth.times.1.0
mm in width.times.0.5 mm in height.
[0107] Then, the Cr--Fe alloy paste 110 including Cr by 60 mass %
and Fe by 40 mass % was printed onto the surfaces of the thermistor
10, which surfaces were opposed to each other in the height
direction thereof, so as to have a thickness of about 30 .mu.m. The
alloy powder had an average grain size of 5 .mu.m as measured by a
laser-diffraction grain size distribution measuring device.
[0108] Then, heat treatment was conducted by performing degreasing
at a temperature of 400.degree. C. and holding the resultant body
for ten minutes at a temperature of 1150.degree. C., while applying
pressure of 30 MPa to the alloy paste 110 formed on the surfaces of
the thermistor 10, using SPS (Spark Plasma Sintering: a sintering
process of applying pressure and current at the time of heat
treatment), thereby obtaining sample X1.
[0109] Sample X0, i.e. a sample of the temperature sensing element
of a comparative example, has a sintered body as the thermistor 10.
The sintered body was prepared by blending 94 vol % of silicon
nitride (Si.sub.3N.sub.4) powder having an average grain size of
0.7 .mu.m, and 6 vol % of yttrium oxide (Y.sub.2O.sub.3) powder, as
a sintering aid, having an average grain size of 0.5 .mu.m,
followed by processes similar to the above.
[0110] Then, an alloy paste serving as the metal electrodes 11 was
printed onto the surfaces of the thermistor 10 through processes
similar to the above. Then, with the resultant body being held,
heat treatment was conducted for ten minutes at a temperature of
1350.degree. C. using hot pressing, while a pressure of 10 MPa was
applied to the alloy paste 110 formed on the surfaces of the
thermistor 10 to achieve bonding between each electrode 11 and the
thermistor 10.
[0111] The samples X1 and X0 were subjected to image analyses using
an annular dark-field scanning transmission electron microscope
(ADF-STEM (using JEM-2100F)) to observe each sample focusing on the
interface between the metal electrode and the thermistor.
[0112] The results are shown in FIGS. 10 to 12.
[0113] FIG. 10 (a) shows an ADF-STEM image of the diffusion layer
of sample X1 of the present invention. FIG. 10 (b) shows analysis
results of an element (Cr) shown by FIG. 10 (a). FIG. 10 (c) shows
analysis results of an element (Fe) shown by FIG. 10 (a). Further,
FIG. 11 is a model diagram illustrating an appearance of a
diffusion layer at a boundary portion between a metal electrode and
the thermistor in sample X1 of the present invention. FIG. 12 is a
model diagram illustrating an appearance of a diffusion layer at a
bonding portion between a metal electrode and the thermistor in
sample X0 of the comparative example.
[0114] As will be understood from FIGS. 10 and 11, sample X1 of the
present invention has the crystal grains 101 of silicon nitride
(Si.sub.3N.sub.4) and the grains 102 of silicon carbide (SiC). The
crystal grain boundary 105 in between the crystal grains is formed
with Fe silicide 107, Fe being the metal element .alpha., and Cr
silicide 106. The Fe silicide 107 and the Cr silicide 106 are
formed reacting with the grains 102 of silicon carbide (SiC) that
are present in the crystal grain boundary 105 in between the
crystal grains 101 of silicon nitride (Si.sub.3N.sub.4). The
diffusion of the Fe silicide 107 and the Cr silicide 106 is
advanced to a depth of some degree from the interface at the metal
electrode 11. On the other hand, as shown in FIG. 11, some
non-diffusion areas 108 (in which no grains of silicon carbide
(SiC) are present and the crystal grain boundary is extremely
narrow) as circled by broken lines are present. However, when the
diffusion reaches to some depth, only the grains 102 of silicon
carbide (SiC) are present in the crystal grain boundary, but
neither the Fe silicide 107 nor the Cr silicide 106 is seen. This
boundary is the interface between the diffusion layer 12 and the
thermistor 10.
[0115] In FIG. 11, the SiC grains 102 are illustrated as being
located on the Fe silicide 107, i.e. the silicide of the metal
element .alpha., and the Cr silicide 106. However, as will be
understood from FIG. 10 by (a), the SiC grains are present being
taken into the Fe silicide and the Cr silicide.
[0116] On the other hand, as shown in FIG. 12, sample X0 of the
comparative example includes the crystal grains 101 of silicon
nitride (Si.sub.3N.sub.4) and the crystal grain boundary 105 in
between. The crystal grains 101 and the crystal grain boundary 105
are formed with the Fe silicide 107, Fe being the metal element
.alpha., and the Cr silicide 106. However, it was found that the Fe
silicide 107 and the Cr silicide 106 were formed being diffused
into only a very small area near the interface of the thermistor
10, with large variation in the state of diffusion. Further, it was
also found that sample X0 of the comparative example subjected to
heat treatment at a temperature of 1350.degree. C. allowed
oxidization of the metal electrodes 11 to increase the resistance
value thereof. On the other hand, it was further found that, when
the temperature of heat treatment was lowered (e.g. 1150.degree. C.
as in sample X1) to mitigate oxidization, diffusion layers were not
formed at all, and thus bonding strength was not ensured.
Experimental Example 2
[0117] First, samples X1 to X10 were prepared for evaluation.
[0118] For sample X1, the following mixture material was prepared
as the thermistor 10. The mixture material was prepared by blending
63.4 vol % of silicon nitride (Si.sub.3N.sub.4) powder having an
average grain size of 0.7 .mu.m, 30 vol % of silicon carbide (SiC)
powder having an average grain size of 0.2 .mu.m, 6 vol % of
yttrium oxide (Y.sub.2O.sub.3) powder, as a sintering aid, having
an average grain size of 0.5 .mu.m, and 0.6 vol % of TiB.sub.2
powder, as metal conductors, having an average grain size of 0.4
.mu.m, followed by mixing for 24 hours with ethanol using a ball
mill
[0119] Then, the mixture material was molded by means of uniaxial
pressing at a pressure of 20 MPa, followed by performing hot
pressing for one hour in a N.sub.2 atmosphere at a temperature of
1850.degree. C. and at a pressure of 20 MPa. Thus, a parallelepiped
(plate-like) sintered body was obtained as the thermistor 10, with
its dimension being 1.0 mm in depth.times.1.0 mm in width.times.0.5
mm in height.
[0120] Then, the Cr--Fe alloy paste 110 including Cr by 60 mass %
and Fe by 40 mass % was printed onto the surfaces of the thermistor
10 so as to have a thickness of about 30 .mu.m, the surfaces being
opposed to each other in the height direction thereof. The alloy
powder had an average grain size of 5 .mu.m as measured by a
laser-diffraction grain size distribution measuring device.
[0121] Then, heat treatment was conducted by performing degreasing
at a temperature of 400.degree. C. and holding the resultant body
for ten minutes at a temperature of 1150.degree. C., while applying
pressure of 30 MPa to the alloy paste 110 formed on the surfaces of
the thermistor 10, using SPS (Spark Plasma Sintering: a sintering
process of applying pressure and current at the time of heat
treatment), thereby obtaining sample X1.
[0122] In the present example, seven more temperature sensing
elements (samples X2 to X8) were prepared changing the metal of the
alloy paste (see Table 1). The temperature sensing elements were
prepared in a manner similar to sample X1 except that the material
of the alloy paste as the metal electrodes 11 was changed.
[0123] The temperature sensing elements were prepared by printing a
paste of Cr powder in sample X2, a paste of Cr-10Ti alloy powder in
sample X3, a paste of Fe-25Cr-5Al alloy powder in sample X4, a
paste of Fe-20Ni-25Cr alloy powder in sample X5, a paste of
Ni-15.5Fe-8.5Cr alloy powder in sample X6, a paste of W powder in
sample X7, and a paste of Pt powder in sample X8.
[0124] Samples X1 to X8 were evaluated as follows.
[0125] Table 1 shows linear expansion coefficient of the metal
electrodes in the temperature sensing elements of the respective
samples. The linear expansion coefficient was measured on the basis
of an isothermal holding measurement method (JIS Z 2285) using a
thermal mechanical analyzer.
[0126] Then, the temperature sensing elements of samples X1 to X8
were evaluated as follows as to bonding reliability, heat
resistance and ohmic contact characteristics.
"Bonding Reliability"
[0127] The samples (samples X1 to X8) were held for two minutes at
a temperature of 1050.degree. C., followed by holding them for two
minutes at a normal temperature (about 25.degree. C.). With this
temperature cycle as being one cycle, the temperature cycle was
repeated for 1000 times (temperature cycle test). Then, the
occurrence of separation of the metal electrodes or the occurrence
of crack generation in the metal electrodes and the thermistor was
observed through a magnification microscope (appearance) and a
metallographic microscope (cross section).
[0128] The samples in which no separation or crack generation was
observed were evaluated with a mark "O". The samples in which a
considerable separation or crack generation was observed were
evaluated with a mark "X". The samples in which separation or
cracks was observed but with a small extent were evaluated with a
mark ".DELTA.". The results are shown in Table 1.
"Heat Resistance"
[0129] The samples (samples X1 to X8) were left standing in a
high-temperature furnace at a temperature of 1050.degree. C. for
500 hours (high-temperature exposure test). After that, the samples
were examined as to the occurrence of melting or oxidization in the
metal electrodes or the diffusion layers through cross-section
observation using a metallographic microscope. The samples in which
no oxidization, melting or the like was observed after the heating
in the high-temperature furnace compared to the state before being
heated were evaluated with a mark "O". The samples in which
considerable melting or oxidization was observed were evaluated
with a mark "X". The samples in which melting or oxidization was
observed but with a small extent were evaluated with a mark
".DELTA.". The results are shown in Table 1.
"Ohmic Contact Characteristics"
[0130] The resistance of the thermistor of each of the samples was
measured before and after conducting the tests of bonding
reliability and heat resistance. Regarding the samples before the
tests, an evaluation with a mark "O" was given to those which had a
resistance variation of 5% or less in the thermistor after being
bonded to the metal electrodes, with respect to the resistance of
the thermistor before being bonded to the metal electrodes. An
evaluation with a mark "X" was given to those which had a
resistance variation of 100% or more. An evaluation with a mark
".DELTA." was given to those which had a resistance variation
exceeding 5% but less than 100%. Further, regarding the samples
after the tests, an evaluation with a mark "O" was given to those
which had a resistance variation of 5% or less in the thermistor,
with respect to the resistance of the thermistor before being
tested. An evaluation with a mark "X" was given to those which had
a resistance variation of 100% or more. An evaluation with a mark
".DELTA." was given to those which had a resistance variation
exceeding 5% but less than 100%. The results are shown in Table
1.
[0131] Evaluation (after the tests) was conducted of either the
resistance variation after the test of bonding reliability
(temperature cycle test), or the resistance variation after the
test of heat resistance (high-temperature exposure test), whichever
was larger.
TABLE-US-00001 TABLE 1 Thermal Ohmic Contact Sam- Expansion
Characteristics Bonding Heat ple Metal Coefficient Before After
Reliabil- Resis- No. Electrodes (.times.10.sup.-6/.degree. C.)
Tests Tests ity tance X1 Cr--Fe alloy 9.0 .largecircle.
.largecircle. .largecircle. .largecircle. X2 Cr 6.5 .DELTA. .DELTA.
.DELTA. .DELTA. X3 Cr--Ti alloy 6.7 .DELTA. .DELTA. .DELTA. .DELTA.
X4 Fe--Cr--Al 11.4 .largecircle. .DELTA. .DELTA. .largecircle.
alloy X5 Fe--Ni--Cr 16 .largecircle. X X X alloy X6 Ni--Fe--Cr 13
.largecircle. X X X alloy X7 W 4.5 X X .largecircle. X X8 Pt 8.8
.largecircle. .DELTA. .largecircle. X
[0132] As shown in Table 1, good evaluation was achieved in all of
bonding reliability, heat resistance and ohmic contact
characteristics by sample X1 which was provided with the metal
electrodes of Cr--Fe alloy that contained Cr and Fe that was a
metal element having a Si diffusion coefficient higher than that of
Cr, and had a linear expansion coefficient of
11.times.10.sup.-6/.degree. C. or less.
[0133] FIG. 6 is a graph collectively indicating heat-resistant
temperature and linear expansion coefficient, based on the
evaluations of the present experimental example, of the metals used
for the electrode and metal silicide layers formed as the diffusion
layer.
[0134] Sample X2 that uses Cr has a linear expansion coefficient
near that of the thermistor (4.5.times.10.sup.-6/.degree. C.) and
thus is considered to mitigate thermal stress. However, as a result
of the temperature cycle test, sample X2 was recognized to cause
separation of the electrodes. This is because, as shown in FIG. 6,
the Cr silicide formed as a diffusion layer has a high linear
expansion coefficient. This means that, Cr silicide does not
exhibit satisfactory bonding reliability unless a suitable
diffusion layer is formed. On the other hand, in the
high-temperature exposure test, Cr as the electrode was oxidized
and thus was found not to exhibit satisfactory heat resistance.
Regarding ohmic contact characteristics as well, Cr of sample X2
does not allow the temperature sensing element to sufficiently
exert its functions due to the variation of the resistance value,
which is ascribed to the separation and oxidation of the
electrodes.
[0135] In this regard, as shown in Table 1, sample X1 that uses a
Cr--Fe alloy is good in all of bonding reliability, heat resistance
and ohmic contact characteristics. This is because, as shown in
FIG. 5, Cr is added with Fe that has a Si diffusion coefficient
smaller than that of Cr and the diffusion layer is formed with not
only the Cr silicide but also the Fe silicide having a low thermal
expansion, and thus the linear expansion coefficient of the
diffusion layer is lowed and oxidization of Cr is suppressed.
Accordingly, in sample X1, the metal electrodes are prevented from
being separated and oxidized. Further, sample X1 is able to
sufficiently ensure ohmic contact characteristics after the
temperature cycle test and the high-temperature exposure test.
[0136] A point of reliably forming a suitable diffusion layer is,
as mentioned above, to add an element having a Si diffusion
coefficient higher than that of Cr. As shown in FIG. 6, Fe has a Si
diffusion coefficient higher than that of Cr. Accordingly, use of a
Cr--Fe alloy as a material of the electrode can reliably ensure
formation of the Fe silicide. On the other hand, addition of Ti
that is an element having a Si diffusion coefficient lower than
that of Cr allows most of the diffusion layer to be formed by the
Cr silicide and thus no suitable diffusion layer is obtained. As a
matter of fact, as will be understood from Table 1, in sample X3
that uses a Cr--Ti alloy, bonding reliability is not sufficiently
ensured.
[0137] As will be seen from Table 1, sample X4 uses a Fe--Cr--Al
alloy, sample X5 uses a Fe--Ni--Cr alloy, and sample X6 uses a
Ni--Fe--Cr alloy. These samples are added with an element (Fe)
having a higher Si diffusion coefficient than Cr. Accordingly, each
of these samples is reliably formed with suitable diffusion layers
and thus exhibits good ohmic contact characteristics before the
tests. However, causing separation in the temperature cycle test,
these samples are not sufficiently satisfactory in bonding
reliability. This is because, in spite of the formation of the Fe
silicide as the diffusion layer, the metal electrode has a high
linear expansion coefficient and thus the generation thermal stress
is not sufficiently reduced. In samples X5 and X6, not only the Fe
silicide but also a Ni silicide is formed as the diffusion layer.
However, as shown in FIG. 6, such a Ni silicide has low
heat-resistant temperature and thus is not able to achieve
satisfactory heat resistance.
[0138] Sample X7 has a linear expansion coefficient that
substantially coincides with the linear expansion coefficient of
the thermistor. Accordingly, sample X7 is imposed with extremely
small thermal stress and thus exhibits satisfactory bonding
reliability. However, as shown in FIG. 6, W that forms the
electrode has a low oxidation resistance temperature. Accordingly,
sample X7 suffered from serious oxidization in the electrode and
thus was not able to exhibit satisfactory heat resistance, thereby
allowing large change in the resistance value.
[0139] Sample X8 using Pt also has a linear expansion coefficient
comparatively approximate to the linear expansion coefficient of
the thermistor. Accordingly, sample X8 has small thermal stress and
thus exhibits satisfactory bonding reliability. However, as shown
in FIG. 6, Pt that forms the electrode has a high heat-resistant
temperature, while a Pt silicide forming the diffusion layer has a
low heat-resistant temperature. Therefore, sample X8 was not able
to achieve satisfactory heat resistance.
[0140] As set forth above, the temperature sensing element of the
present invention (sample X1) was found to ensure heat resistance
and bonding reliability under high temperature conditions. At the
same time, sample X1 was found to have good ohmic contact
characteristics and enable substantially balanced and stable
temperature detection.
Experimental Example 3
[0141] In the present experimental example, characteristics changes
of temperature sensing elements are discussed, in the case where
the metal electrode has a different mixing ratio of Cr and Fe. In
the present experimental example, as will be shown in Table 2
discussed later, seven different temperature sensing elements
(samples X9 to X15) were prepared using alloy powders having a
different mixing ratio of Cr and Fe. The samples were each prepared
in a manner similar to sample X1 described above, except that the
material of an alloy paste forming the metal electrode 11 is
different.
[0142] The temperature sensing elements were prepared by printing a
paste of Cr-5Fe alloy powder in sample X9, a paste of Cr-10Fe alloy
powder in sample X10, a paste of Cr-25Fe alloy powder in sample
X11, a paste of Cr-40Fe alloy powder in sample X12, a paste of
Cr-55Fe alloy powder in sample X13, a paste of Cr-70Fe alloy powder
in sample X14, and a paste of Cr-85Fe alloy powder in sample
X15.
[0143] Similar to experimental example 1, the samples (samples X9
to X15), after their linear expansion coefficient being measured,
were evaluated as to bonding reliability, heat resistance and ohmic
contact characteristics.
[0144] The evaluations are shown in Table 2. FIG. 13 shows a
relationship of rate of oxidation increase (%) before and after a
high-temperature exposure test (evaluation of heat resistance),
with respect to Fe-mixing ratio (mass %) in a Cr--Fe alloy. The
rate of oxidation increase was calculated using the weight of each
of the samples measured before and after the high-temperature
exposure test, on the basis of an equation:
Rate of oxidation increase=(weight after test-weight before
test)/weight before test
TABLE-US-00002 TABLE 2 Thermal Ohmic Expansion Bonding Heat Contact
Sample Metal Coefficient Reli- Resis- Characteristics No.
Electrodes (.times.10.sup.-6/.degree. C.) ability tance (After
Tests) X9 Cr--5Fe 6.9 .largecircle. .DELTA. .DELTA. X10 Cr--10Fe
7.2 .largecircle. .largecircle. .largecircle. X11 Cr--25Fe 8.1
.largecircle. .largecircle. .largecircle. X12 Cr--40Fe 9
.largecircle. .largecircle. .largecircle. X13 Cr--55Fe 9.8
.largecircle. .largecircle. .largecircle. X14 Cr--70Fe 11
.largecircle. .largecircle. .largecircle. X15 Cr--85Fe 11.7 .DELTA.
.DELTA. .DELTA.
[0145] As will be understood from Table 2, samples X10 to X14 using
a Cr--Fe alloy that contained Fe in the metal electrodes by 10 to
70 mass % obtained satisfactory evaluations in all of the items. On
the contrary, as will also be understood from Table 2, sample X9
using Cr-5Fe alloy and sample 15 using Cr-85Fe allowed lowering of
at least heat resistance. This is because, as shown in FIG. 13, the
metal electrodes are oxidized when a Fe-mixing ratio in the Cr--Fe
alloy is less than 10 mass % and more than 70 mass %.
[0146] Further, as will be understood from Table 2, sample X15
using Cr-85Fe alloy caused slight separation of the metal
electrodes in the temperature cycle test. This is because Cr-85Fe
alloy has a high linear expansion coefficient and thus is not able
to sufficiently reduce thermal stress. On the other hand, sample
X14 using Cr-70Fe alloy ensures bonding reliability. Accordingly,
it is considered that, with a linear expansion coefficient being
11.times.10.sup.-6/.degree. C. or less, thermal stress is
sufficiently reduced and satisfactory bonding reliability is
exerted.
[0147] In the present experimental example, oxygen-resistivity
enhancing effects were examined, in the case where Al was mixed
into Cr-40Fe alloy. The results are shown in FIG. 14.
[0148] As will be seen from FIG. 14, addition of Al by 0.5 mass %
or more can suppress oxidation increase to thereby enhance heat
resistance of the electrodes. Further, addition of Al by more than
7 mass % does not show any change in the effects but tends to
deteriorate processability. Accordingly, an Al-mixing ratio may
preferably be 7 mass % or less. FIG. 14 exemplifies Cr-40Fe. It was
confirmed that the Cr--Fe alloys having a Fe-mixing ratio ranging
from 10 mass % (Cr-10Fe alloy) to 70 mass % (Cr-70Fe alloy) showed
similar results. It should be appreciated that not only heat
resistance but also bonding reliability and ohmic contact
characteristics are sufficiently satisfactory.
Experimental Example 4
[0149] Here, fifteen different temperature sensing elements
(samples X16 to X30) are prepared to examine their characteristics
change. These samples have a different thickness in the metal
electrode and a different thickness in the diffusion layer.
[0150] Specifically, samples X16 to X30 were obtained in a manner
similar to experimental example 1 with a change in the amount of
coating of a paste of alloy powder and the time of heating the
paste so that the thickness of the metal electrode and the
diffusion layer would have a value as shown in Table 3 which will
be discussed later. The temperature sensing elements of the samples
(samples X16 to X30) were prepared in a manner similar to sample X1
of experimental example 1 except that the thickness of the metal
electrode and the diffusion layer was changed.
[0151] A thickness t1 of the metal electrode 11 and a thickness t2
of the diffusion layer 12 can be confirmed through the observation
using a scanning electron microscope (SEM) (see (a) of FIG. 15
described later). Similar to experimental example 1, the samples
(samples X16 to X30) were evaluated as to bonding reliability, heat
resistance and ohmic contact characteristics.
[0152] The results are shown in Table 3.
TABLE-US-00003 TABLE 3 Thickness Thickness Ohmic of Metal of
Diffusion Bonding Heat Contact Sample Electrode Layer Reli- Resis-
Characteristics No. (.mu.m) (.mu.m) ability tance (After Tests) X16
1 to 2 1 to 2 .DELTA. X X X17 3 3 .largecircle. .largecircle.
.largecircle. X18 11 6 .largecircle. .largecircle. .largecircle.
X19 12 18 .largecircle. .largecircle. .largecircle. X20 28 9
.largecircle. .largecircle. .largecircle. X21 31 28 .largecircle.
.largecircle. .largecircle. X22 62 35 .largecircle. .largecircle.
.largecircle. X23 56 50 .largecircle. .largecircle. .largecircle.
X24 103 15 .largecircle. .largecircle. .largecircle. X25 101 32
.largecircle. .largecircle. .largecircle. X26 106 63 .largecircle.
.largecircle. .largecircle. X27 108 101 .largecircle. .largecircle.
.largecircle. X28 120 55 .DELTA. .largecircle. .DELTA. X29 122 115
.DELTA. .largecircle. .DELTA. X30 148 128 X .largecircle. X
[0153] As shown in Table 3, bonding reliability was insufficient in
sample X16 having the electrode with a thickness of 1 to 2 .mu.m
and the diffusion layer with a thickness of 1 to 2 .mu.m. This is
because, the metal electrode having a metal electrode coating of an
excessively small thickness had insufficient strength which, being
coupled with thermal stress, caused cracks in the electrode. Sample
X16 does not have fully satisfactory heat resistance, either. This
is because the excessively small thickness of the metal electrode
allowed advancement of oxidization from the surface of the metal
electrode to the thermistor. As a result, the resistance value
became extremely large and thus ohmic contact characteristics were
not ensured any more.
[0154] On the other hand, in sample X28, the metal electrode has a
large thickness exceeding 110 .mu.m. Also, in samples X29 and X30,
the metal electrode and the diffusion layer have a large thickness
exceeding 110 .mu.m. These samples exhibit satisfactory heat
resistance but do not exhibit fully satisfactory bonding
reliability. This is because the stress generated on the bonding
interface was increased, thereby inducing cracks in the
thermistor.
[0155] On the other hand, in samples X17 to X27, the metal
electrode and the diffusion layer have a thickness of 3 to 110
.mu.m. These samples exhibited satisfactory ohmic contact
characteristics without causing any problem such as of separation,
crack generation or oxidization after the temperature cycle test
and the high-temperature exposure test.
[0156] Then, the temperature sensing element of sample X19 was
examined as to the conditions of the bonding interface between the
metal electrode and the thermistor. Specifically, the bonding
interface of sample X19 was observed using a scanning electron
microscope (SEM). The results are shown in FIG. 15 (a). Further,
energy dispersive X-ray fluorescence analysis (EDX) was conducted
of Si, Cr and Fe. The analyses of Si, Cr and Fe are shown in FIGS.
15 (b), (c) and (d), respectively.
[0157] As will be seen from FIG. 15 (a), in the bonding interface
of sample X19, the diffusion layer 12 is formed, achieving
diffusion bonding between the metal electrode 11 and the thermistor
10.
[0158] Further, as will be seen from FIGS. 15 (b) to (d), Si as a
component of the thermistor and Cr and Fe as components of the
metal electrode are diffused in the bonding interface between the
metal electrode 11 and the thermistor 10 to form a metal silicide
composed of the Cr silicide and the Fe silicide.
[0159] The present invention is not limited to the embodiments
described above but may be modified in various manners without
departing from the spirit of the present invention.
EXPLANATION OF REFERENCES
[0160] 1, 90: Temperature sensing element [0161] 10, 91: Thermistor
[0162] 11, 92: Metal electrodes [0163] 12: Diffusion layers [0164]
3, 95: Sheath pin [0165] 33: Insulating member [0166] 34: Outer
tube member [0167] 301: Tip end portion [0168] 4: Cover [0169] 5:
Temperature sensor [0170] 21, 93: Signal lines [0171] 93: Signal
lines [0172] 12: Rib [0173] 121: Contact portion [0174] 122:
Rearward extension portion [0175] 123: Forward extension portion
[0176] 13: Protective tube [0177] 101: Crystal grains of silicon
nitride [0178] 102: Grains of silicon carbide [0179] 103: Metal
conductors [0180] 105: Crystal grain boundary composed of
crystallized glass phase
* * * * *